This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Background: Cushing's disease refers to a condition of glucocorticoid exceess caused by an adrenocorticotropic hormone (ACTH) producing pituitary tumor, which account for 10-15% of all pituitary tumors. The majority of corticotroph tumors are microadenomas at the time of diagnosis, and accurate surgical and histologic identification of these tumors can be challenging. ACTH is produced in corticotroph cells within the anterior pituitary via the precursor pro-opiomelanocortin (POMC). In both physiologic and pathologic conditions the promoter for POMC is regulated by multiple transcription factors which include AP-1 and Nurr77. Retinoic acid has been shown to inhibit activation of the POMC promoter in corticotroph tumor cell culture via disruption of Nurr77 transcriptional activity. The expression of the orphan nuclear receptor termed chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI) antagonizes retinoic acid signaling, and has been reported to be present in normal corticotroph cells, but lacking in adenomatous corticotroph cells in tissue culture studies. Through the retrospective analysis of 34 human corticotroph tumors we have demonstrated a consistent lack of COUP-TFI in 100% of the microadenomas that were not visible, or measured less than 5 millimeters by preoperative MRI. In total, 85% of all tumors studied showed absence of COUP-TFI. Based on in vitro data from rat and human corticotroph tumors, cells lacking COUP-TFI are vulnerable to retinoid-induced cell death via Nurr77-mediated apoptosis, an effect that is reversed by COUP-TFI gene transfection. In 2006, Castillo et al. published the results of a six-month trial which randomized 44 dogs with Cushing's disease to an RXR agonist (9-cis retinoic acid), or to ketoconazole. RXR agonist therapy outperformed ketoconazole for all endpoints, resulting in normalization of ACTH and cortisol levels in 100% of subjects that completed the study, and improved morbidity and mortality. All of the dogs treated with the RXR agonist remained in remission for the duration of the 6 to 12 month post-treatment followup. Hypothesis and Objectives: We hypothesize that the differential expression of COUP-TFI in normal vs. adenomatous corticotroph cells in human microadenomas identifies a retinoid-sensitive tumor population which is vulnerable to retinoid-induced ACTH-suppression and apoptosis. The objective of this pilot study is to establish the safety and tolerability of short-term therapy with bexarotene in patients with Cushing's disease, and study the clinical, biochemical, and cellular effects of a preoperative five-day course of bexarotene in these patients before undergoing transsphenoidal surgery. The primary biochemical outcome measures will be diurnal plasma ACTH and cortisol, and serial 24-hour urine free cortisol levels. Methods: This pilot study (n = 6 subjects) will involve inpatient admission to our General Clinical Research Center for 5 days prior to scheduled transsphenoidal surgery. During the five days of the study each individual will receive the RXR-agonist bexarotene at the FDA approved dose of 300 mg/m2/day. Clinical signs and symptoms of acute adrenal insufficiency will be monitored routinely throughout each 24-hour period. Baseline and twice-daily biochemical analysis for ACTH and cortisol will be performed. 24-hour urine collection for cortisol will be obtained pre-treatment and in the last 24-hours of treatment. Laboratory safety analysis will include serial comprehensive metabolic panels to monitor liver and kidney function, complete blood count to monitor for neutropenia, as well as thyroid function studies to monitor for central hypothyroidism which can develop with therapy.